def climb_stairs(n: int) -> int:
    """
    其实就是斐波那契数列
    爬一级: 1种；(1)
    爬两级: 2种;(1+1, 2)
    爬三级: 3种；(1+1+1, 1+2, 2+1)
    推出f(n) = f(n-1) + f(n-2)
    :param n:
    :return:
    """
    a = b = 1
    for i in range(2, n + 1):
        a, b = b, a + b
    return b
